1. Subject of the Invention
The subject invention is generally directed to nozzles and, more particularly, nozzles suitable for atomizing high viscosity slurries.
2. Description of the Prior Art
In recent years, there has been increased interest in the use of fuel slurries; that is, a mixture of powdered solid fuel suspended in a liquid. The liquid may be combustible, such as oil; or incombustible, such as water. In addition, the slurry may contain additives that tend to maintain the solids in suspension and retard settling. In either case, it has been found preferable to maximize the relative solid content of the mixture. Thus, slurry mixtures are characterized by high viscosity.
For example, coal slurries have been formed wherein powdered coal is suspended in water. A typical coal/water slurry contains up to about 70% by weight of coal that has been screened to a particle size of about 200 micrometers. The coal particles have varying mineral content and are generally abrasive.
At the time of combustion, the slurry fuel must be atomized such that it is dispersed and mixed with air in a manner similar to the atomization of liquid fuels. Furthermore, if the suspension liquid is noncombustible, such as water, it must be evaporated before the solid fuel particles can be burned.
For many years, various devices such as spray-drying towers have been used to spray and disperse slurries. However, these devices used a rotating-disc or wheel that was motor driven and were, therefore, unsuitable for use in combustion applications.
Many types of nozzles for atomizing low-viscosity liquid fuels are known in the prior art. For example, various nozzles have been used to atomize petroleum-based liquid fuels for combustion in a furnace or boiler. Basically, many such liquid atomizers accelerate the liquid to a high velocity and interact it with a gas such as air or steam. The resulting turbulence disrupts the liquid stream into small particles. Other liquid atomizers atomize low viscosity liquid fuels such as kerosene by pressurizing the liquid and forcing it through a small orifice or swirl chamber. However, such prior nozzles were found to be sensitive to the viscosity of the liquid fuel so that they were not well suited for use with high-viscosity slurries.
In atomizing relatively high-viscosity liquid fuels such as heavy petroleum distillates or residual oils, it has been generally necessary to use a different nozzle wherein high-pressure air or steam is used to accelerate the liquid fuel. In addition, the high-viscosity liquid fuels are also sometimes preheated.
Because of the abrasive nature of slurry particles, such high-viscosity liquid fuel atomizers are generally unsuited for use in atomizing a slurry. In many such high-viscosity nozzles, the fuel and gas interact inside the atomizer. Thus, the fuel is accelerated to high velocities inside the atomizer. Since the solid fuel particles of slurries, such as a coal/water slurry tend to be abrasive, the use of such nozzles with slurries allowed the accelerated particles to scrub the internal surfaces of the atomizer. This resulted in rapid erosion of the nozzle. Thus, there was a need in the prior art for an atomizer that was not sensitive to the viscosity of the slurry or subject to rapid erosion by the slurry particles.